The present invention relates generally to fiber optic cables, and more particularly to a novel fiber optic cable and method of making the same wherein one or more fiber optic elements are disposed within a nonmetallic tubular shield over which a nonmetallic braided strength member is coaxially formed in tight fitting relation so that an axial load applied to the cable is taken by the braided strength member so as to protect the fiber optic element from undesirable elongation and damage.
The use of low-loss fiber optic cables has found wide acceptance in many industrial and commercial applications, including the fields of computer technology and communications. Low-loss fiber optic cables offer many desirable advantages over metallic conductors including use for long distance transmission without repeaters, immunity from cross talk, greater bandwidth capabilities, lighter weight, and potential for lower cost signal communication systems.
As a practical matter, it has been found that significant problems are encountered which inhibit wide utilization of the desirable features of fiber optic cables over metallic conductors. One basic problem is the inherent fragility of glass fibers which makes more difficult the production of fiber optic cables which are flexible and can withstand bending, twisting, impact, vibration, etc. The basic approach to solving this problem has been to provide means for strengthening and buffering the individual fiber optic elements so that subsequent bundling, cabling and field usage will not damage or adversely affect the optical properties of the fiber optic elements.
As a result of the fragility of the glass fibers employed in fiber optic cables, the glass fibers are capable of withstanding only relatively low elongation per unit length when subjected to tensile loading. Some fiber optic elements will fail by breaking when subjected to approximately 1% elongation while other types of fiber optic elements can withstand approximately 6% elongation without failure. Because the fiber optic elements are capable of withstanding only relatively low elongation, it is known to provide a nonmetallic axial strength member within a fiber optic cable to carry the fiber optic elements so that the axial strength member takes the major portion of any tensile load applied to the cable.
In addition to employing axial strength members within fiber optic cables to prevent the fiber optic elements from being subjected to potentially damaging tensile loads, it is also known to employ nonmetallic strands of suitable strength material externally along the length of a bundle of one or more fiber optic elements in a fiber optic cable to improve the tensile strength characteristics of the cable without damage to the fiber optic elements. In general, however, the nonmetallic strands are positioned in parallel relation to the longitudinal axis of the cable along the length of the cable. One problem with this technique for preventing damaging tensile loading of the fiber optic elements is that during the manufacturing process, some lengths of the reinforcing strands may be longer than other lengths. This results when the fiber optic cable is curved around one or more pulleys or sheaves during manufacture with the result that the high strength strands adjacent the shorter radius of curvature have slightly shorter length than the strands adjacent the outer or larger radius of curvature. When the resulting fiber optic cable is subjected to a tensile load, the shorter reinforcing strands are subjected to greater tensile loading than the longer strands and may fail, thereby causing failure of the fiber optic cable.